| Plants are anchored in soil via their roots.Roots play a central role in plant growth and development by absorbing water and nutrients from soil and sensing various surrounding environmental cues.As branches of plant primary roots,the development of lateral roots is essential for determining the architecture and function of the plant root system in dicots,and is tightly controlled by cell-to-cell and cell-to-environment communications.Receptor-like protein kinases(RLKs),a group of single-pass transmembrane proteins,have been reported to regulate multiple aspects of plant growth,development and immunity.Although some studies on the functions of RLKs in regulating lateral root development have been reported,the detailed molecular mechanisms remain elusive.Genes of a number of uncharacterized RLKs were found to be expressed in lateral root.Functional analyses of these new RLKs will contribute our better understanding of internal and external factors in regulating lateral root development.These studies may provide new strategies for crop improvement in future agriculture.Previously,our group had constructed the promoter::GUS transgenic plants for all 223 leucine-rich repeat receptor-like protein kinases(LRR-RLKs)in Arabidopsis.Our detailed analysis indicated that one of the LRR-RLKs,RLK68,is specifically expressed in lateral roots.Interestingly,RLK165,the closest paralog of RLK68,is also mainly expressed in lateral roots.Double mutant rlk68 rlk165,generated from the null single alleles of RLK68 and RLK165,displays a phenotype with significantly reduced number of lateral roots.Genetic complementation and the consistent phenotype of the second independent double mutant rlk68-2 rlk165-2 confirmed that the lateral root defects are indeed caused by the deletions of both RLKs.Detailed cellular analysis indicated that some lateral roots of double mutant rlk68 rlk165 are maintained at stage I during the lateral root development.Early development defects of the lateral root cause the decreased number of mature lateral roots.Subcellular localization analysis indicated that RLK68 is polarly distributed in the primary root and lateral root cells,which is similar to that of PIN family proteins.Whereas RLK165 is mainly identified in the primary root endodermis cells and the lateral root primordia.Exogenous auxin application can enhance the expression of RLK68,but the location and expression of auxin responsive reporter DR5,auxin efflux carrier PIN1,PIN3 and auxin influx carrier AUX1 in double mutant rlk68 rlk165 remain unchanged.These results suggested that RLK68 may play a role downstream of auxin.Phosphorylation assay showed that RLK68 and RLK165 are not active kinases in vitro but can be phosphorylated by other kinases in planta,indicating that they are atypical kinase-inactivated receptor kinases.Meanwhile,the physical interaction of RLK68 and RLK165 revealed by coimmunoprecipitation experiments suggested that they may function as a complex.In addition,primary roots and lateral roots of double mutant rlk68 rlk165 are hypersensitive to the treatment of CEP5,a peptide hormone known to regulate primary and lateral root growth and development.However,this hypersensitivity of rlk68 rlk165 to CEP5 does not depend on CEPR1,putative receptor of CEP5,suggesting that RLK68 and RLK165 function independently to CEPR1 related signaling pathway.RNA-Seq data and quantitative real-time PCR(qRT-PCR)results showed that the transcriptional levels of several cell wall degradation genes are significantly down-regulated in rlk68 rlk165,suggesting that RLK68 and RLK165 affect the development of lateral roots via regulating cell wall remodeling.In conclusion,by forming complex with other kinase,RLK68 and RLK165 may sense specific ligands to regulate early lateral root development through modulating cell wall remodeling. |
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